Have just re-read the BFL's post on P30L. I thought he wrote about adjustable Vabs that I somehow missed.
Hopefully I got it right this time, correct me if I'm wrong. It has only ONE adjustable voltage for everything.
You set it to 15V - you want your batts to get to 15V daily, obviously. So you are sitting and watching it, taking turns with your wife, until it gets to 15V. Then after you've made sure it stayed 40-60 minutes at 15V, you re-set it to 13.6V Float and can go fishing, finally. Ops - too late, fishes have already gone.